Intraocular lenses of the type described herein are used primarily to treat problems of vision in a human eye.
A normal eye has two lenses with four refracting surfaces. Both lenses are convergent lenses which tend to focus light rays inward toward an axis of the eye. Greatest refraction takes place at the air-cornea interface, and lesser refraction occurs at the cornea-aqueous humour interface. Refraction again occurs at the aqueous humour-crystalline lens interface and again at the crystalline lens-vitreous humour interface. The aqueous humour and vitreous humour have approximately the same refractive index which is slightly below the refractive indices of the cornea and crystalline lens. The crystalline lens is composed of the capsule, the cortex and the nucleus, all of these surfaces having different indices of refraction allowing for color correction for chromatic aberrations due to refractions of incoming wavelengths of light.
When a crystalline lens is removed because of occlusion, for example with cataracts, its function may be replaced by a relatively thick converging lens worn outside of the eye. Preferably, in removing the crystalline lens the capsule is held intact to maintain separation between the vitreous humour and the aqueous humour.
In recent times, an intraocular lens is surgically positioned in the posterior chamber of the eye behind the iris and in front of the capsule. The intraocular lens, called a pseudophakos, is held in place with a haptic or other mount which extends outward from the lens and engages outer walls of the capsule or the sulcus. In another mounting method, the lens may be attached directly to the iris or to the chamber walls or within the cornea by any known means.
The pseudophakos has the advantage of eliminating the requirement for thick eyeglasses and increasing the field of vision, as compared with thick eyeglasses which may be used to replace a removed crystalline lens.
Techniques are known for anchoring lenses within the eye.
When a normal eye gazes upon an object, the cornea and crystalline lens focus an object upon a part of the retina which is in optical alignment with those lenses and which is called the macula, which joins the retina with the optic nerve and which is most sensitive to light.
A common problem which may develop, following cataract surgery, is the difficulty with correct color perception due to the inability of current lenses to correctly and concurrently refract the color wavelengths of light. An achromatic lens improves the color correction problem and thus improves overall image quality.
Current intraocular lenses are made of glass, plastics, silicone or hydrogel, polycarbonate or other suitable materials.
The use of a single element lens will allow for a focus of some wavelengths of light, but will cause a blur for other wavelengths. Some lenses produce false or distorted colors or color separation or fringing of images, known as chromatic aberration.
The construction of a conventional achromatic lenses is well known. Fine controlled forming, molding and grinding, matching spherical and non-spherical concave, convex or planar surfaces on lenses and joining exact opposite surfaces together with bonding material are known in the art.
Glass, such as combined lenses of flint and ground glass chosen for their different refractive indices, may be joined together as an achromatic lens, or plastics having different indices of refraction may be joined together or may be combined with glass of a desired index of refraction. One plastic well known for use in intraocular lenses is methyl methacrylate. Other suitable materials are known, such as polycarbonate, hydrogel, glass of silicone.
The inventor has created, and has a U.S. Pat. No. 5,201,762 for, an achromatic intraocular lens for use in the system made of two optical lens components joined together to form a doublet. The two lenses are preferably made of materials having different refractive indices. Refraction occurs at the interface as well as at the distal and proximal surface of the joined multiple lenses. Provided the appropriate degree of magnification or relocation of the image on the retina may be accomplished by the achromatic lens, that lens is a suitable multiple intraocular lens system.
One basic plastic may be used and impregnated with materials which provide differing indices of refraction in separate lens elements of the achromatic lens. The use of an achromatic lens consisting of two materials of different refractive indices allows for better color correction than lenses previously described.
The inventor continuously seeks to improve intraocular lenses. A need continues to exist for other chromatic aberration correcting intraocular lenses that may meet different requirements of patients.